Fiberglass-reinforced plastics (FRP) composites are finding increased usage in a variety of end-use applications. For examples, FRP composites are increasingly being used in ships, especially Naval ships, as load-bearing structures, such as light weight foundations, deckhouses and masts.1 Industrial uses for FRP composites include piping, valves, centrifugal pumps, heat exchangers as well as gratings, screens and ventilation ducts. Such increased usage is being driven by a number of market needs, for example, so as to reduce maintenance, lower weight, increase covertness (especially for Naval warships) and decrease costs. FRP composites must however be sufficiently fire resistant so that the composite is not a source of spontaneous combustion and will not contribute to the rapid spread of fire. Generally, flame retardants are incorporated in a FRP composite to achieve the desired flame resistance. 1 U. Sorathia, J. Ness, M. Blum, “Fire safety of composites in the US Navy”, Composites: Part A, 30, 707-713 (1999).
Flame retardants interfere with burning by acting either through the vapor phase or the condensed phase by chemical and/or physical mechanisms. Some common types of flame retardants and mechanisms of action include:2 2Lu et al, “Recent developments in the chemistry of halogen-free flame retardant polymers”, Prog Polym Sci, 27, 1661-1712 (2002).                Fillers—act to dilute the polymer and reduce concentration of decomposition gases;        Hydrated fillers—release non-flammable gases or decompose endothermically to cool the pyrolysis zone at the combustion surface;        Halogen, phosphorus and antimony—act in vapor phase by a radical mechanism to interrupt the exothermic processes and to suppress combustion;        Phosphorus—also acts in the condensed phase promoting char formation acting as a barrier to inhibit gaseous products from diffusing to the flame and shields the polymer from heat and air; and        Intumescent materials—materials swell when exposed to fire or heat to form a porous foamed mass acting as a barrier to heat, air and pyrolysis products.        
In FRP composite materials, fillers and halogenated resins are the most common methods used to achieve flame resistance. Fillers such as aluminum trihydrate release water upon heating. However, such fillers have to be incorporated in high amounts and have a negative effect on mechanical properties. Halogenated resins have clear disadvantages, particularly, the toxicity of hydrogen halide formed during combustion.3 Toxic fumes released during the combustion of halogenated resins can be lethal in the confined spaces found in aircraft fuselages or marine hull compartments. 3 Id.
Flame retardants can be incorporated into polymeric materials either as additives or as reactive materials. Additive type flame retardants are widely used by blending with polymeric materials. In FRP resins, the flame retardant additive is added to the resin prior to fiber impregnation. Additives present problems including poor compatibility, leaching and reduced mechanical properties. Reactive flame retardants are an attempt to overcome the problems of additives through copolymerization of the flame retardant with the polymer. Copolymerized flame retardants are designed not to leach or reduce mechanical properties. At this time, most copolymerized flame retardants are based on halogenated monomers with the aforementioned problems of toxicity.
Recently, in U.S. Pat. No. 6,290,887 to Sheu et al (the entire content of which is expressly incorporated hereinto by reference), superabsorbent polymer (SAP) particles pre-loaded with moisture have been incorporated into a thermoplastic polymer (e.g., polyethylene) so as to obtain a SAP-enriched plastics material that may be extruded into desired shapes (e.g., as an outer jacket of a telecommunications cable).
It would therefore be desirable if flame-retardant SAP could be incorporated in curable thermoset resins without adversely affecting the resin curing process. It would especially be desirable if such flame-retardant SAP could be reacted with curable thermoset resins during the curing process so that, when cured, the SAP would be chemically bound (linked) to the polymeric chain of the resulting cured thermoset resin. In such a manner, improvements to flame retardant properties as well as improvements to other mechanical/physical properties (e.g., impact resistance) could be “engineered” into FRP composites formed of such thermoset resins. It is towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is embodied in products and processes whereby flame-retardant SAP particles are incorporated into synthetic resins, especially curable thermosettable resins. The SAP particles are most preferably hydrated with an aqueous flame-retardant solution. In this regard, the flame-retardant solution may consist essentially of water alone or a water solution containing one or more water soluble inorganic flame retardants.
When SAP particles are hydrated with an aqueous inorganic flame retardant solution, the SAP particles may thereafter be dried to remove substantially the water component. In such a manner, the inorganic flame retardant will remain as a dried residue physically entrained within the SAP particles. As such, the SAP particles serve as a physical matrix in which the inorganic flame retardant is homogenously dispersed. The SAP particles may then be blended with a synthetic resin as is or alternatively may be ground into more finely divided particles which contain the dried residue of the aqueous inorganic flame retardant solution and then blended with a suitable synthetic resin.
It has been found that, when incorporated into a curable thermoset resin and cured, the fame-retardant SAP particles do not affect the curing process of the thermoset resin. The SAP particles may also be modified so as to include one or more pendant reactive groups which serve to react with the thermoset resin during the curing process so as to be chemically bound (linked) thereto.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.